Convalidation.

After the generation you can double check that the domain is correct. A little more text improves the design too, imho. Ex. "nostr.com has been shortened to:".

Another thing: I would not use the same input element for user input and place to put the result; it is a UX useful when there are space constraints, here you have a lot of room and you can keep the input on the bottom for the next conversion.